Turbopropeller, or “turboprop” engines may be employed on aircraft to provide thrust necessary for flying. The turboprop engine typically consists of a gas turbine engine, or gas generator, burning air and fuel to generate an exhaust gas flow that drives an aerodynamically-coupled power turbine, or propeller turbine, which is further coupled to a set of propeller blades. In this sense, the speed of the gas generator indirectly affects the speed of the power turbine by providing torque to drive the propeller, thus providing thrust.
The pitch of the propeller blades may also be rotated on the propeller shaft to provide additional thrust at the expense of increasing the torque demand of the turbine shaft. For instance, if the pitch of the propeller blades is reduced, each blade rotates on the propeller shaft such that the air resistance of the blade as it rotates about the shaft decreases, and thus, reduces the torque demand for the shaft. In the instance where the gas generator speed is constant, the reduced torque demand of the propeller results in increased propeller shaft RPMs. Conversely, if the pitch of the propeller blades is increased, each blade rotates on the propeller shaft such that the air resistance of the blade as it rotates about the shaft increases, and thus, increases the torque demand for the shaft. In the instance where the gas generator speed is constant, the increased torque demand of the propeller results in decreased propeller shaft RPMs.
Turboprop control systems may employ dual control levers used by the pilot to adjust, respectively, the gas generator speed demand, i.e. the speed of the gas turbine engine, and the propeller speed demand, i.e. via the pitch of the propeller blades. This dual-lever control system allows for variation of aircraft speed and torque demand during, for instance, takeoff, cruise, and/or reverse thrust operations.